A type of zoom lenses, which have specifications of approximately 70 mm in focal length in the wide-angle view mode, of approximately 3× in zoom ratio, and of 2.8 in F number unvaried during the zooming, are generally comprised of first to fourth groups of lenses, that is, the 1st lens group of positive refractive power, the 2nd lens group of negative refractive power, the 3rd lens group of positive refractive power, and the 4th lens group of positive refractive power (see Patent Documents 1 and 2 listed below). Some of this type of the zoom lenses well-known in the art have only the 2nd and 3rd lens groups moved for the zooming while the 1st and 4th lens groups are stationarily halting; and they have the 1st lens group moved for the focusing (i.e., the foremost-lens focusing) or only part of the 1st lens group is moved for the focusing, which is specifically called “the foremost-lens inner-focusing.”
For the purpose of downsizing the foremost-lens inner-focusing zoom lens, developed and proposed has been an optics architecture where the focusing group of lens pieces are moved even during the zooming (see Patent Document 3 listed below).
Patent Document 1 Japanese Patent No. 3266653
Patent Document 2 Japanese Patent No. 3491136
Patent Document 3 Japanese Patent No. 3397440
In the Patent Documents 1 and 2, the foremost-lens inner-focusing zoom lens, which is capable of inhibiting effects of aberration more in comparison with the foremost-lens focusing zoom lens, has its minimum photo-shooting distance ranging approximately from 1.5 meters to 1.4 meters. With the minimum photo-shooting distance reduced down to 1 meter, lens pieces are accordingly to be displaced greater for the focusing to resultantly elongate the entire lens extension. For the purpose of minimizing a diameter of the focusing group of lens pieces, the leading part and the trailing part of the 1st lens group have their respective refractivities determined so that the leading part is greater in refractivity than the trailing part. As a consequence, spherical aberration is significantly insufficiently compensated during the focusing for the macro-photographing in the telephoto view mode to resultantly bring about a degradation in performance, and the focusing for the maximized close-up photographing results in an image having its peripheral area affected by eclipse.
In the Patent Document 3, for the same purpose of reducing a diameter of the focusing group of lens pieces, the focusing lens group is moved even for the zooming, and this is prone to cause beams to be shaded to make the focused image eclipsed.
In general, the foremost-lens inner-focusing where the 1st lens group is functionally divided into the leading set of lens pieces and the trailing set of lens pieces dedicated to the focusing has been widely incorporated in large-aperture-ratio telephoto zoom lens designs. With such a large-aperture-ratio telephoto zoom lens, a displacement of the 1st lens group must be increased to take a picture at a shorter object distance, and accordingly the entire extension of the first lens group is elongated. The trailing set of lens pieces, if they have an increased refractivity for the purpose of reducing the displacement of the 1st lens group, become greater in curvature of radius and have a larger thickness to ensure a sufficient lens diameter, and this should unavoidably cause the lens to be enlarged in radial directions. If the lens optics is downsized by an approach that the remaining lens groups other than the 1st lens group have their respective refractivities increased, greater effects of spherical aberration are adversely caused, and additionally a curvature of field is exaggerated.
The present invention is made to overcome the aforementioned disadvantages in the prior art large-aperture-ratio inner-focusing telephoto zoom lens, and accordingly, it is an object of the present invention to provide a compact large-aperture-ratio inner-focusing telephoto zoom lens of the reduced total lens extension where minimizing a photo-shooting distance down to as short as 1 meter does not cause the lens optics to displace accordingly greater during the focusing.
It is another object of the present invention to provide a large-aperture-ratio inner-focusing telephoto zoom lens that is capable of sufficiently compensating for spherical aberration during the focusing for the macro-photographing in the telephoto view mode without having a resultant image affected by eclipse during the focusing for the maximized close-up photographing.
It is still another object of the present invention to provide a large-aperture-ratio inner-focusing telephoto zoom lens that can avoid shading beams as a result of moving the focusing lens during the zooming so as not to make the focused image eclipsed.